Xenopus leavis is being studied as a vertebrate animal that offers certain advantages for the analysis of molecular events during development. The major approach currently underway is based on the preparation and analysis of an enriched cDNA library which represents those RNA molecules that are present in gastrulae but absent from the egg. These sequences account for a few percent of the gastrula poly(A) RNA and represent those genes that have been turned on for the first time in the blastula embryo. Through a novel methodological approach for the cloning of enriched cDNA we obtained a library representing gastrula differentially expressed (DE) sequences. Some DE RNAs accumulate very rapidly, implying synthetic rates comparable to those of fully induced differentiated genes. Most gastrula DE RNAs appear to be mRNAs since they are found preferentially loaded on polysomes. By the tadpole stage most gastrula DE mRNAs have decreased substantially in abundance, suggesting that they may be specifically involved in early embryogenesis. The most prominent DE cDNA, clone 42, has been sequenced, and genomic DNA corresponding to this gene family is being isolated. A repetitive DNA element with certain properties of a transposable sequence has been isolated from the Xenopus genome. The 1723 element has a length of 6 to 10 kb, and its 8500 very similar copies are interspersed in the genome. Calmodulin coding sequences have been isolated from Xenopus by cDNA cloning. In contrast to the situation in the chicken which contains a single calmodulin gene (A. Means and co-workers, personal communication) Xenopus carries two such genes. Both are apparently active since two distinct cDNA clones have been isolated, which have 95% nucleotide homology but encode the identical calmodulin protein. We are testing whether the two genes are differentially or coordinately expressed in Xenopus development.